With the advances in microscopic manufacturing technology, much attention has been focused on so-called micro-machine (MEMS: Micro Electro Mechanical Systems, ultra-miniature electric, mechanical compound) devices and miniature devices in which MEMS devices are incorporated.
A MEMS device is a device that is formed as a microscopic structure on a substrate, with an insulation film, which is formed on a semiconductor substrate made of silicon (Si), gallium arsenic (GaAs), or the like; or on an insulative substrate such as a glass substrate, a quartz substrate, or the like; in which a driving body outputting mechanical driving force and a semiconductor integrated circuit or the like that controls the mechanical body are electrically united. A basic feature of the MEMS device is such that a mechanically structured driving body is incorporated into a part of the device, and the driving body is electrically driven by the use of coulombic attraction force or the like between electrodes.
FIGS. 13 and 14 show typical compositions of an optical MEMS device that is applied to an optical switch and a light modulation device by taking advantage of the reflection or diffraction of light.
An optical MEMS device 1 shown in FIG. 13 includes a substrate 2, a substrate side electrode 3 formed on the substrate 2, a beam 6 having a driving side electrode 4 that is disposed in parallel to oppose the substrate side electrode 3, and a support part 7 for supporting one end of the beam 6. The beam 6 and substrate side electrode 3 are electrically insulated by a void 8 therebetween.
A required substrate, such as a substrate with an insulation film formed on a semiconductor substrate of, for example, silicon (Si), gallium arsenic (GaAs) and the like or an insulative substrate such as a glass substrate or a quartz substrate is used for the substrate 2. The substrate side electrode 3 is formed of a polycrystalline silicon film by doping impurities therein, metal film (Cr deposited film, for example), and the like. The beam 6 is composed of, for example, an insulation film 5 such as silicon nitride film (SiN film) or the like and the driving side electrode 4 serving as a reflective film consisting of, for example, Al film of about 100 nm in thickness. The beam 6 is formed in a so-called cantilever fashion with its one end supported by the support part 7.
In the optical MEMS device 1, the beam 6 displaces itself in response to electrostatic attraction force or electrostatic repulsion force generated between the substrate side electrode 3 and driving side electrode 4 by an electric potential that is applied to the substrate side electrode 3 and driving side electrode 4, and as shown by a solid line as well as broken line in FIG. 13, for example, the beam 6 displaces itself into a parallel state or inclined state relative to the substrate side electrode 3.
An optical MEMS device 11 shown in FIG. 14 is composed of a substrate 12, a substrate side electrode 13 formed on the substrate 12 and a beam 14 that straddles the substrate side electrode 13 in a bridge-like fashion. The beam 14 and substrate side electrode 13 are insulated by a void 17 therebetween.
The beam 14 is composed of a bridge member 15 of, for example, a SiN film, that rises up from the substrate 12 and straddles a substrate side electrode 13 in a bridge-like fashion and a driving side electrode 16 of, for example, an Al film of about 100 nm in thickness, that, serving as a reflective film, is provided on the bridge member 15 to oppose the substrate side electrode 13 in parallel to each other. The substrate 12, substrate side electrode 13, beam 14 and others may employ similar compositions and materials those explained in FIG. 13. The beam 14 is formed in a so-called bridge-like fashion in which the both end thereof are supported.
In the optical MEMS device 11, the beam 14 displaces itself in response to electrostatic attraction force or electrostatic repulsion force generated between the substrate side electrode 13 and driving side electrode 16 by an electric potential that is applied to the substrate side electrode 13 and driving side electrode 16, and as shown by a solid line and a broken line as well in FIG. 14, for example, the beam 14 displaces itself into a parallel state or fallen state relative to the substrate side electrode 13.
With these optical MEMS devices 1, 11, light is irradiated on the surfaces of the driving side electrodes 4, 16 serving as a light reflective film, and by taking advantage of differences in the direction of reflected light depending upon positions into which the beams 6, 14 are driven, these MEMS devices can be applied to an optical switch having a switch function of detecting the reflected light in one direction.
Further, the optical MEMS devices 1, 11 are applicable as a light modulation device for modulating the strength of light. When light reflection is taken advantage of, the strength of light is modulated by vibrating the beams 6, 14 according to the amount of reflected light in one direction per unit time. This light modulation device runs on a so-called time modulation.
When light diffraction is taken advantage of, a light modulation device is composed of a plurality of beams 6, 14 disposed in parallel relative to the common substrate side electrodes 3, 13, and by varying the height of, for example, driving side electrodes each serving as a light reflective film with the movements of every other beam 6, 14 such as moving closer to or moving away from the common substrate side electrodes 3, 13, the strength of reflected light from the driving side electrodes is modulated by means of light diffraction. This light modulation device runs on a so-called space modulation.
FIGS. 15 and 16 show a conventional method for manufacturing a MEMS device. Those figures show the case in which the method is applied to the above-mentioned MEMS device 11 having the beam in bridge fashion, which is illustrated in FIG. 14.
First, as shown in FIG. 15A, a single-crystalline silicon substrate 31 is prepared. Next, as shown in FIG. 15B, an insulation film 32 is formed on a main surface of the substrate 31 to insulate the substrate from a substrate side electrode which is formed later on. For example, a thermal oxide film (SiO2 film) 32 is formed by heat treatment at 950° C. in the oxygen atmosphere.
Then, as shown in FIG. 15C, on the insulation film 32 a conductive film 33 is formed, which becomes the substrate side electrode formed of, for example, a poly-crystalline silicon film or a non-crystalline silicon film. For example, after forming an intrinsic poly-crystalline silicon film or intrinsic non-crystalline silicon film using silane (SiH4), hydrogen (H2) gas by means of a low-pressure CVD method, an activating process is conducted by ion implantation or by thermal diffusion, of phosphor (P). Alternatively, the conductive film 33 is directly formed by adding phosphine (PH3) when forming a film.
Next, as shown in FIG. 15D, the conductive film 33 is subjected to patterning so as to form a substrate side electrode 13.
Then, as shown in FIG. 15E, a thermal oxide (SiO2) film 34 is formed on the entire surface so as to cover the substrate side electrode 13. The thermal oxide film 34 is, for example, formed by heat treatment at 950° C. in the oxygen atmosphere. At this time, crystal growth of poly-crystalline or non-crystalline silicon of the underlaid substrate side electrode 13 is accelerated, so that the surface unevenness of the thermal oxide film 34 becomes large.
Next, as shown in FIG. 16A, a sacrificial layer 35 of, for example, a non-crystalline silicon film for forming a void is formed on the overall surface of the thermal oxide film 34.
Next, as shown in FIG. 16B, the sacrificial layer 35 is subjected to patterning such that a portion corresponding to the substrate side electrode 13 is left intact while the other portion is removed.
Next, as shown in FIG. 16C, a bridge member 15 made of, for example, a silicon nitride (SiN) film is formed, followed by selectively forming a drive side electrode 16 made of, for example, Al on the portion corresponding to the substrate side electrode 13 of the bridge member 15 so as to form a bridge-like beam 14.
Next, as shown in FIG. 16D, the sacrificial layer 35 is removed so as to form a void 17 between the substrate side electrode 13 and beam 14, and consequently an electrostatic drive type MEMS device 11 is obtained.
Ultimately, the MEMS device 11 has come to have, as relative roughness of the film surface of the drive side electrode 16 of the beam 14, the surface relative roughness reflecting all of the unevenness at the time of forming the substrate side electrode 13, the unevenness that grew at the time of forming the thermal oxide film 34, and the unevenness of the uppermost surface of the Al film (drive side electrode 16) itself.
FIG. 17 shows a composition of a GLV (Grating Light Valve) device developed by SLM (Silicon Light Machines) as a light strength modulation device for a laser display, that is, as a light modulator.
In a GLV device 21, as shown in FIG. 17A, a common substrate side electrode 23 made of a Cr thin film is formed on an insulative substrate 22 such as a glass substrate or the like, and a plurality of beams 24, in this example, six beams 24 [241, 242, 243, 244, 245, 246] straddling across the substrate side electrodes 23 in a bridge-like fashion are disposed in parallel. The compositions of the substrate side electrode 23 and beams 24 are the same as those explained in above-described FIG. 14. Namely, as shown in FIG. 17B, a reflective film cum driving side electrode 26 made of an Al film of about 100 nm in thickness is formed on the surface, which is parallel to the substrate side electrode 23, of a bridge member 25 made of a SiN film, for example.
The beam 24 made of the bridge member 25 and reflective film cum driving side electrode 26 provided thereon is a portion so-called a ribbon.
The aluminum film (Al film) used as the reflective film cum driving side electrode 26 is a suitable metal as the material for optical components because of the following reasons: (1) it is a metal that can be comparatively easily formed into a film; (2) the dispersion of reflectance with respect to wavelengths in a visible light range is small; (3) alumina natural oxidation film generated on the surface of the Al film functions as a protective film to protect a reflective surface.
Further, the SiN film (silicon nitride film) composing the bridge member 25 is a SiN film formed by means of the low-pressure CVD method, and the SiN film is selected by reason of the physical values of its strength, elasticity constant, and the like being suitable for mechanically driving the bridge member 25.
When a voltage is applied between the substrate side electrode 23 and reflective film cum driving side electrode 26, the above-mentioned beam 24 moves closer to the substrate side electrode 23 according to the above-mentioned electrostatic phenomenon, and when the application of the voltage is stopped, the beam 24 moves away from the substrate side electrode 23 and returns to an original position.
The GLV device 21 alternately varies the height of the reflective film cum driving side electrode 26 with the movements of the plurality of beams 24 (that is, those of every other beams) such as moving closer to or moving away from the substrate side electrode 23, and modulates the strength of light reflected on the driving side electrode 26 by means of the diffraction of light (one beam spot is irradiated on the whole of six beams 24).
Mechanical characteristics of the beam driven by taking advantage of electrostatic attraction force and electrostatic repulsion force are almost predicated on the physical properties of the SiN film formed by the use of the CVD method or the like, with an Al film mainly functioning as a mirror.
Then, it is extremely important, for the optical MEMS device that takes advantage of the reflection and diffraction of light, to control the relative roughness (RMS) of the film surface of the light reflective film cum driving side electrode. Because, the device's characteristics deteriorate as light reflection and light diffraction efficiencies decline according to film surface unevenness.
The substrate side electrode of the MEMS device is formed on a required substrate, such as a substrate with an insulation layer formed on a semiconductor made of, for example, silicon, GaAs, or the like, or an insulative substrate like a quartz substrate or a glass substrate, for example. As for materials thereof, a poly-crystalline film or a metal film in which impurities are doped is conventionally used. However, at a time of forming these films, crystalline grains are generated and as a consequence, there occurs unevenness representing the thickness of the film and crystalline grains on the surface.
For example, a poly-crystalline silicon film is conventionally formed by means of the chemical vapor deposition method (CVD method); however, it has been known that the size of crystalline grains greatly differs from about several tens nm to about several μm depending on the temperature of atmosphere. When a film of 300 nm in the thickness is formed at 650° C. by the use of, for example, the low-pressure CVD method, crystalline grains of 500 nm to 1 μm or so are generated, and at that time the relative roughness (RMS) value of the surface of the substrate side electrode becomes more than 10 nm. The relative roughness of the surface of the substrate side electrode sequentially affects upper layers in the subsequent film forming process and transcribed to the film surface of the optically important driving side electrode in an expanded fashion. Ultimately, a light reflective film cum driving side electrode having piled-up surface unevenness is manufactured.
When formed under the condition of low temperature below 600° C., a non-crystalline silicon without crystallinity is obtained. In this case, there is virtually no relative roughness caused by a single film, though in the subsequent process, when conditions under high temperature are required, for example, at processes: for forming a thermal oxide film, for activation after impurities are doped, for forming a silicon nitride (SiN) film and the like, non-crystalline silicon crystallizes and becomes a poly-crystalline silicon having crystalline grains. When a plurality of processes are conducted at high temperatures, crystalline growth is accelerated, so that on the surface ultimately the film having unevenness affected by underlaid crystalline layers may be formed.
As mentioned above, when the substrate side electrode is formed of poly-crystalline silicon, surface unevenness is, as shown in FIG. 18, increased and transcribed to the surface of the driving side electrode (Al film) 4 constituting the beam (Al/SiN laminated layer) 6, resulting in the deterioration of the light reflectance of the driving side electrode 4 functioning as a mirror.
For example, with the MEMS device in which aluminum (Al) is made to serve as a reflective film, if the film is an ideal bulk Al film, reflectance of the Al film obtained may possibly be about 92%. However, when a poly-crystalline silicon film is formed as the substrate side electrode, followed by forming an Al film on the uppermost surface, that is to become a light reflective film cum driving side electrode, after the process of, for example, forming a thermal oxide film or a SiN film at high temperature, the reflectance of the beam surface will deteriorate by more than several percentage points, so that there is a case in which only about 85% of the reflectance of the beam surface can barely be obtained. Therefore, as one of the measures to improve the characteristics of the MEMS device, there has been great demand for technology to planarize the relative roughness of the surface, particularly, of the underlaid substrate side electrode.